In a cellular mobile radio communication system, it is known to control transmitted power as a function of a power reference coming from the system itself, and in particular as a function of the distance of a mobile from an element of the infrastructure, also referred to as a base station, with which the mobile is in communication, for the purpose of reducing the overall level of interference in the system.
In a digital mobile radio communication system using the time division multiple access (TDMA) technique, it is also known to control the transmitted power as a function of the waveform of the signal to be transmitted. More precisely, since the information to be transmitted is put into the form of packets or "bursts", for transmission during predetermined time intervals in a frame structure, the waveform has a level corresponding to the working portion of the packet, preceded by a rising slope and followed by a falling slope, the two slopes corresponding to guard times on either side of the working portion.
In order to power a power amplifier in such a manner as to perform such control over transmitted power, it is known to provide an active three-pole power device for controlling said power amplifier, the active three-pole device receiving not only a control voltage as a function of the power to be transmitted, but also a power supply voltage for the active three-pole device, and delivering power supply current to the power amplifier at a given power supply voltage for said power amplifier. Said power supply voltage for the active three-pole device is also generally fixed and thus set, in practice, as a function of the maximum power that is to be transmitted.
The main drawback of such a circuit is poor efficiency at low transmitter power since the product of said power supply current multiplied by the voltage drop across the terminals of the active three-pole device can then rise to relatively high values.
To avoid that drawback, and in the context of an analog mobile radio communication system, it is known from document EP 0 626 765 to supply said power supply current and said amplifier power supply voltage via a voltage converter, itself including a controlling active three-pole device which is controlled as a function of the power to be transmitted, but which does not itself deliver said power amplifier power supply voltage and current in full, but delivers only that portion of said power supply voltage and current which varies as a function of the demanded transmitter power.
Such a solution is unsuitable for digital mobile radio communication systems of the kind outlined above, or more generally for any system in which transmitter power is likely to vary relatively quickly, i.e. too quickly relative to the reaction times necessary for such a voltage converter, due to the presence therein of components such as inductors and capacitors having relatively long time constants. To give an order of magnitude, in the mobile radio communication system known under the abbreviation GSM (for "Global System, for Mobile Communication"), the signals transmitted are liable to vary during said rising and falling slopes, by 70 dB over a time interval of about 30 microseconds.